Real-time normalization apparatus and method of phase generated carrier demodulation in sinusoidal phase modulation interferometer

Abstract

The present invention discloses a real-time normalization apparatus and method of the PGC demodulation in a sinusoidal phase modulation interferometer. An optical setup containing a measuring interferometer and a monitoring interferometer is constructed. An electro-optic phase modulator is placed in the common reference arm of the two interferometers. High-frequency sinusoidal wave modulation and low-frequency triangular wave modulation are applied to the electro-optic phase modulator at the same time. Sinusoidal modulation is used for generating phase carrier, and PGC demodulation is performed to obtain quadrature signals containing the phase information to be measured. Triangular wave modulation makes the quadrature signals change periodically. Ellipse fitting is performed on the Lissajous figure corresponding to the quadrature signals, and real-time normalization of the PGC demodulated quadrature signals is achieved. By calculating the variation of the phase difference between the two interference signals, the measured displacement is obtained, and nanometer scale displacement measurement is achieved.

Description

BACKGROUND OF THE INVENTION1. Field of the Invention[0001]The present invention belongs to the technical field of laser interferometric precision measurement, and particularly relates to a real-time normalization apparatus and method of phase generated carrier (PGC) demodulation in a sinusoidal phase modulation interferometer.2. Description of Related Art[0002]The sinusoidal phase modulation interferometry applies sinusoidal modulation on the optical path difference of an interferometer, and it adopts the phase generated carrier (PGC) demodulation to obtain the phase information of an interference signal. Because it has the advantages of simple phase modulation, high measurement accuracy, strong anti-interference ability, etc., the sinusoidal phase modulation interferometry has become an important high-precision measurement technique and widely used in the measurements of displacement, vibration, surface topography, micro angles and so on. The PGC demodulation has the beneficial eff...

AI Technical Summary

Benefits of technology

The present invention is about a new optical setup that uses a measuring interferometer and a monitoring interferometer. The setup includes an electro-optic phase modulator that is placed in the common reference arm of both interferometers. The phase modulator is modulated with both high-frequency sinusoidal and low-frequency triangular signals. The sinusoidal modulation is used to generate the phase carrier, while the triangular modulation is used to change the phase of the quadrature signals periodically. The changes in the quadrature signals are then measured, and the measured displacement is obtained by calculating the variation of the phase difference between the two interference signals. The invention also avoids the use of polarizing optics, which helps to eliminate non-linear errors caused by polarization errors. The optical setup is simple and convenient to use and is suitable for sub-nanometer accuracy in various fields such as ultra-precision fabrication, micro-electronics manufacturing, precision metrology, etc.

Problems solved by technology

But the output light intensity of the laser diode is also modulated, which will introduce the amplitude modulation and result in a phase demodulation error.

The algorithm of the DCM method is more complicated, and is prone to affecting by the light intensity change and the interference fringe contrast.

However, in PGC demodulation, phase delay caused by the optical path transmission, drifting of the phase modulation depth, changes of the gains of multipliers or filters or the like may result in a nonlinear error.

However, this method is not applicable to a static object such as the absolute distance measurement.

In addition, during the measurement, the normalization coefficients may change due to the drifting of the phase modulation depth or the change of the measured displacement, so the normalization coefficients obtained by off-line estimation is not precise any more, and therefore the measurement accuracy is lowered.

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